The power factor of an AC electric power system is defined as the ratio of the real power (voltage and current in phase) flowing to a load, to apparent power (voltage and current out of phase), and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 power factor=50% power factor). Real power is the capacity of a circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Energy stored in the load and returned to the source, or non-linearities in the load that distort the wave shape of the current drawn from the source, often cause the apparent power to be greater than the real power. A load with low power factor draws more current than a load with a high power factor for the same amount of useful power transferred, and thereby causes higher resistive losses in wiring. It is therefore desirable to correct power factor for many types of load.
Non-linear loads, such as rectifiers, distort the current drawn from the system into a non-sinusoidal waveform. Non-linear loads require active power factor correction to counteract the distortion and raise the power factor. Power factor correction may occur within equipment at a central substation, within equipment throughout a distribution system, or may be performed within power-consuming equipment.
A typical switched-mode power supply, as found in many consumer products, first powers a DC bus, using a bridge rectifier or similar circuit. The output voltage is then derived from this DC bus. Since rectifiers are non-linear devices, the input current is highly non-linear and has a low power factor resulting from energy at harmonics of the frequency of the voltage. Regulatory agencies such as the EU have set harmonic limits as a method of improving power factor. Declining component cost has hastened implementation of power factor correction. To comply with current EU standard EN61000-3-2, all switched-mode power supplies with output power more than 75 W must include power factor correction (PFC). 80 PLUS power supply certification requires power factor to be corrected to 0.9 or greater.
To achieve a higher power factor, Active Power Factor Correction (active PFC) is used to control the amount of power drawn by a load, in order to obtain a power factor as close as possible to unity. In most applications, the active PFC controls the input current of the load so that the current waveform is proportional to the mains voltage waveform (a sine wave).
Some types of active PFC are: Boost circuits, Buck circuits, and Buck-boost circuits, and may be implemented as single-stage or multi-stage. In the case of a switched-mode power supply, an active PFC circuit may use a boost converter inserted between the bridge rectifier and the main input capacitors. The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Typically, a second switched-mode converter inside the power supply produces the desired output voltage from the DC bus voltage. This approach requires additional semiconductor switches and control electronics, but permits using cheaper and smaller passive components than passive PFC. Switched-mode power supplies with passive PFC can achieve power factor of about 0.7-0.75, whereas switched-mode power supplies with active PFC, may achieve a power factor up to 0.99. Without PFC, switched mode power supplies typically have a power factor of about 0.55-0.65.
FIG. 1 shows one exemplary prior art power device 100 with controlled output power and power factor correction (PFC). Power device 100 is shown driving a load 114. A first section 102 of device 100 implements PFC and a second section 104 provides an isolated output voltage 108 through a transformer 106. In this example, an integrated circuit NCP1603 facilitates PFC within first section 102 and includes a pulse width modulation (PWM) circuit to implement the secondary switched-mode power conversion, within second section 104, as commonly used in power supply devices.
In this example, load 114 operates at a voltage 112 that is provided by a voltage regulator 110 which uses an output voltage 108 of second section 104. Second section 104 operates in a switched-mode to generate voltage 108 from transformer 106. Second section 104 includes optical feedback to the integrated circuit which operates to maintain voltage 108 irrespective of current drawn by load 114 and voltage supplied by first section 102. At startup of device 100, first section 102 operates to produce an operating voltage 105 to supply second section 104. To avoid startup problems where second section 104 overloads first section 102 when attempting to provide voltage 108, and hence voltage 113 to load 114, the integrated circuit typically delays the start of second section 104, for between 0.5 and 3 seconds, to allow first section 102 to attain operating voltage 105. Where load 114 represents a lighting application, such startup delay is undesirable.
Further, in this example, output voltage 108 of device 100 may contain ripple from second section 104, since second section 104 operates by generating an alternating current through transformer 106.
As shown, device 100 includes voltage regulator 110 to reduce voltage 108 to voltage 112 as required by load 114. Where voltage 112 is varied to control operation of load 114 (i.e., voltage regulator 110 operates to vary voltage 112), power loss in the form of dissipated heat from voltage regulator 110 may be undesirable. For example, using the simple equation of “watts=amps*volts”, where voltage 108 is 20V and current drawn by load 114 is 1 A at 10V, power dissipation by voltage regulator 110 is 10 W, which result in an efficiency of only 50% (since power used by load 114 is 10 W) or less for device 100.
In particular, where voltage 112 supplied to load 114, and hence current through load 114, varies, efficiency of device 100 is dependent on the voltage drop across, and current through, voltage regulator 110. The greater the voltage drop across the regulator, the greater the power loss and the lower the efficiency.
An issue currently confronting LED manufacturers and the LED lighting industry is the sensitivity of human perception to the properties of LED light, and the difficulty of precise process control in LED manufacturing such that spectral differences among LEDs are not objectionable in lighting products. At the present time, LED manufacturers and the LED lighting industry are working together to identify and segregate LEDs with specific spectral properties such that end users can select appropriately “warm” or “cool” LED lighting, and so that mixtures of LEDs with differing spectral properties do not present a nuisance or distraction within a fixture or across fixtures in an installation. It is typical for LED lighting manufacturers to carefully order LEDs from single LED manufacturer batches and to track them for use in particular light fixture orders. The present necessity to do so can have negative implications for inventory management and production scheduling—that is, it is expensive and/or risky to build “to stock” because product can become useless if the product built does not include a specific batch of LEDs needed for a future order.